The Cantor gets its own portal; bullets stop strobing

A dungeon now names the boss arena it ends in, and the arena decides the boss.
Three entrances in the hub: Warden's Descent, The Choir Vault, Proving Grounds.

That replaces the seed coin-flip from the last commit, which was the wrong
call. Which boss you are about to fight is the single thing a player decides
before walking into a dungeon, and rolling it for them makes that decision
unavailable. Only the Proving Grounds still leaves its arena open, because a
harness you re-enter every couple of minutes wants whichever fight comes up
first -- and it is the one place where "either will do" is true.

test_every_boss_has_a_dungeon_that_reaches_it is the check that matters here:
adding a boss and forgetting to give it a way in is now a failing test rather
than a boss nobody meets. The smoke test asserts all three portals open over a
real socket -- bots pick theirs by account id, so a run exercises each.

Bullets no longer animate. The sheet's eight frames are a COLOUR cycle rather
than a shape change, so running it had every bullet on screen strobing through
a palette in unison, which with a few hundred in the air is exactly as hard to
look at as it sounds. Each bullet holds one frame and turns slowly instead,
offset by its own id so a ring of twenty does not rotate as one rigid wheel.
The rate is slow enough that nothing completes a full turn inside its own
lifetime, so it reads as drift rather than spin, and a test pins that against
the longest-lived bullet in the game. The renderer slices four textures at
startup now instead of thirty-two.

check.sh clean, 414 tests, SMOKE PASS (22 assertions), all four diagnostics
green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/content/rooms.gd

@@ -112,17 +112,17 @@ static func stamp(grid: MapGrid, s: PackedStringArray, origin: Vector2i) -> Dict
 ##   P  a dungeon portal   S  player spawn   T  practice target
 ##   U  the upgrade NPC
 ##
-## Each `P`, in reading order, opens the matching entry in Dungeons.ORDER. Two
-## of them now: the real run and the Proving Grounds, side by side so they can
-## be compared without restarting anything. Adding a third dungeon means adding
-## a third marker here.
+## Each `P`, in reading order, opens the matching entry in Dungeons.ORDER: the
+## Warden's Descent, the Choir Vault, and the Proving Grounds. Adding a dungeon
+## means adding an entry there and a marker here, and they have to stay in the
+## same order.
 static func lobby() -> PackedStringArray:
 	return PackedStringArray([
 		"#########################################",
 		"#.......................................#",
 		"#.......................................#",
 		"#.......................................#",
-		"#..............P.........P..............#",
+		"#.........P.........P.........P.........#",
 		"#.......................................#",
 		"#.....o...........................o.....#",
 		"#.......................................#",